A magnetic memory, especially a magnetic random access memory (MRAM) operates as a nonvolatile memory capable of a high-speed operation and rewriting an infinite number of times. Therefore, some types of MRAMs have been put into practical use, and some types of MRAMs have been developing to improve their general versatility. In the MRAM, a magnetic material is used as a memory element, and data is stored in the memory element as a magnetization direction of the memory element. Some methods for switching the magnetization direction of the memory element are proposed. Those methods have in common with usage of a current. To put a MRAM into practical use, it is important to reduce the writing current as much as possible. According to the non-patent literature 1 (N. Sakimura et al., “MRAM Cell Technology for Over 500-MHz SoC”, IEEE Journal of Solid-State Circuits, vol. 42, p. 830 (2007)), it is required that the wiring current should be reduced to be equal to or less than 0.5 mA, preferably equal to or less than 0.2 mA. This is because the minimum layout can be applied to the 2T-1MTJ (Two transistors—One Magnetic tunnel junction) circuit configuration proposed in the non-patent literature 1 to realize the cost performance equal to or more than that of the existing volatile memory.
The most general method of writing data in a MRAM is to switch a magnetization direction of a magnetic memory element by a magnetic field which is generated by passing a current through a wiring line for a writing operation prepared on the periphery of the magnetic memory element. Since this method uses a magnetization switching caused by the magnetic field, the MRAM can theoretically perform writing at a speed of 1 nano-second or less and thus, the MRAM is suitable for a high-speed MRAM. However, a magnetic field for switching magnetization of a magnetic material securing thermal stability and resistance against external disturbance magnetic field is generally a few dozens of [Oe]. In order to generate such magnetic field, a writing current of about a few mA is needed. In this case, a chip area is necessarily large and power consumed for writing increases. Therefore, this MRAM is not competitive with other kinds of random access memories. In addition, when a size of a memory cell is miniaturized, a writing current further increases and is not scaling, which is not preferable.
Recently, as methods to solving these problems, following two methods are proposed. The first method is a method using a spin transfer magnetization switching. This method uses a lamination layer including a first magnetic layer (magnetization free layer) which has magnetization that can be switched, and a second magnetic layer (reference layer) which is electrically connected to the first magnetic layer and has magnetization that is fixed. In the method, the magnetization in the first magnetic layer (magnetization free layer) is switched by using an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer (magnetization free layer) when a current flows between the second magnetic layer (reference layer) and the first magnetic layer (magnetization free layer). A reading operation is carried out by using a magnetoresistive effect generated between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer). Therefore, the MRAM using the spin transfer magnetization switching method is an element having two terminals. The spin transfer magnetization switching is generated when a current density is equal to or more than a certain value. Accordingly, as the size of the element decreases, the writing current is also reduced. In other words, the spin transfer magnetization switching method is excellent in scaling performance. However, generally, an insulating film is provided between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer) and a relatively large current should be made to flow through the insulating film in the writing operation. Thus, there are problems regarding resistance to writing and reliability. In addition, there is concern that a writing error occurs in the reading operation because a current path of the writing operation is the same as that of the reading operation. As mentioned above, although the spin transfer magnetization switching method is excellent in scaling performance, there are some obstacles to put it into practical use.
On the other hand, the second method, which is a magnetization switching method using a current induced domain wall motion effect, can solve the above-mentioned problems that the spin transfer magnetization switching method is confronted with. For example, a MRAM using the current induced domain wall motion effect is disclosed in the patent literature 1 (Japanese patent publication JP2005-191032A). Regarding the above MRAM using the current induced domain wall motion effect, in the first magnetic layer (magnetization free layer) having the magnetization which can be switched, generally, magnetization of both end portions is fixed such that the magnetization of one end portion is approximately anti-parallel to that of the other end portion. In the case of such magnetization arrangement, a domain wall is introduced into the first magnetic layer. Here, as reported in the non-patent literature 2 (A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, Physical Review Letters, vol. 92, p. 077205 (2004)), when a current flows through the domain wall, the domain wall moves in the direction same as the direction of the conduction electrons. Therefore, the data writing can be realized by making the current flow inside the first magnetic layer (magnetization free layer). The data reading is realized by using the magnetoresistive effect caused by a magnetic tunnel junction provided in a region where the domain wall moves. Therefore, the MRAM using the current induced domain wall motion method is an element having three terminals, and fits in the 2T-1MTJ configuration proposed in the above-mentioned non-patent literature 1. The current induced domain wall motion is generated when the current density is equal to or more than a certain value. Thus, this MRAM has the scaling property similar to the MRAM using the spin transfer magnetization switching. In addition, in the MRAM element using the current induced domain wall motion, the writing current does not flow through the insulating layer in the magnetic tunnel junction and the current path of the writing operation is different from that of the reading operation. Consequently, the above-mentioned problems caused in the spin transfer magnetization switching can be solved.
Meanwhile, in the non-patent literature 2, a current density of approximately 1×108 A/cm2 is required for the current induced driven domain wall motion. For example, it is assumed that a width and a thickness of a layer where the domain wall motion arises are 100 nm and 10 nm, respectively. In this case, the writing current is 1 mA. This cannot satisfy the above-described condition for the writing current. However, as described in the non-patent literature 3 (S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, Journal of Applied Physics, vol. 103, p. 07E718 (2008)), it is reported that, by using a material having perpendicular magnetic anisotropy as a ferromagnetic layer (magnetization free layer) where the current induced domain wall motion arises, the writing current can be sufficiently reduced. Because of this, in the case of manufacturing an MRAM using the current induced domain wall motion, it is preferable to use a ferromagnetic material having perpendicular magnetic anisotropy as a layer (magnetization free layer) where the domain wall motion arises.
As a related art, Japanese patent publication JP2004-153248A (corresponding to U.S. Pat. No. 7,218,484 (B2)) discloses a magnetoresistive effect element, a manufacturing method of the same, a magnetic head and a magnetic reproducing device. In the magnetoresistive effect element includes a magnetoresistive effect film, a pair of electrodes and a phase separating layer. The magnetoresistive effect film includes: a first ferromagnetic layer whose magnetization direction is substantially fixed in one direction; a second ferromagnetic layer whose magnetization direction is varied depending on an external magnetic field; and a middle layer formed between the first and second ferromagnetic layers. The pair of electrodes is electrically connected to the magnetoresistive effect film such that it can make a sense current flow in a direction approximately perpendicular to a film surface of the magnetoresistive effect film. The phase separating layer includes a first phase and a second phase into which alloy composed of a several kind of elements is separated within a solid phase. One of the first phase and the second phase contains at least one element selected from a group consist of oxygen, nitrogen, fluorine and carbon with high density. The phase separating layer is formed between the pair of electrodes.
Japanese patent publication JP2005-209251A discloses an initializing method of a magnetic memory. In the initializing method of a magnetic memory, the magnetic memory includes: magnetic memory elements in each of which a memory layer holding information by a magnetization state of a magnetic material is composed of a plurality of magnetic layers; and, first wiring lines and second wiring lines intersecting with each other, wherein the magnetic memory elements are provided near intersection points where the first wiring lines and the second wiring lines intersect with each other. In the initializing method, by stopping of applying current pulses to the first wiring lines and current pulses to the second wiring lines at approximately the same time, magnetization stats of the memory layers of the magnetic memory elements are made to be in the same state.
Japanese patent publication JP2008-147488A discloses a magnetoresistive effect element and a MRAM. The magnetoresistive effect element includes: at least two of first magnetization fixed layers whose magnetization direction is fixed; a magnetization free layer whose magnetization direction is variable and which is formed on a first plane; and a second magnetization fixed layer which is connected to the magnetization free layer through a non-magnetic layer and whose magnetization direction is fixed. The two first magnetization fixed layers are arranged facing the second magnetization fixed layer through the magnetization free layer and are magnetically coupled with the magnetization free layer. Both of the magnetization of the two first magnetization fixed layer has a component in a first direction perpendicular to the first plane. In a data writing operation, a writing current is made to flow from one end of the magnetization free layer to the other end within the first plane.